1. Field of the Invention
The present invention relates to an exhaust pipe for an engine and, more specifically, the present invention relates to a double walled exhaust pipe having an outer wall and an inner wall.
2. Description of the Related Art
A double walled exhaust pipe is generally used to connect an exhaust manifold of an engine to a catalytic converter in the exhaust system. Catalytic converters can remove the pollutants in the exhaust gas of the engine only when the temperature of the catalyst is high. Therefore, the double walled exhaust pipe is used to keep the exhaust gas temperature high by preventing the heat dissipation through the exhaust pipe wall The double walled exhaust pipe usually composed of two metal pipes arranged coaxially with a radial clearance therebetween. The inner pipe forms the inner wall of the double walled exhaust pipe and the exhaust gas from the engine flows through the inner pipe. The outer pipe forms the outer wall of the double walled exhaust pipe, the air in the radial clearance between the inner and the outer walls acts as a insulating layer to prevent heat dissipating from the exhaust gas in the inner pipe to the atmosphere.
In the double walled exhaust pipe, the temperature of the inner pipe wall becomes high when the engine is in operation since the inner pipe wall contacts the hot exhaust gas directly, while the temperature of the outer pipe wall is kept relatively low.
Due to the difference of the temperatures between the inner pipe and the outer pipe, the amount of the thermal expansion of the inner pipe can be larger than the that of the outer pipe.
To prevent this difference in the amounts of the thermal expansion from causing stress in the exhaust pipe, Japanese Unexamined Utility Model Publication No. 55-127828 discloses a construction of a double walled exhaust pipe which is capable of compensating for the difference in the thermal expansion between the inner and the outer pipes.
The double walled exhaust pipe in the Japanese Unexamined Utility Model Publication No. 55-127828 is provided with an outer pipe which is divided into a plurality of longitudinal pipe sections, and sliding connections which connect the pipe sections of the outer pipe. The sliding connection permits the relative slide movement between the pipe sections along the longitudinal direction while restricting the radial relative movement between the pipe sections. In the double walled exhaust pipe disclosed in the above publication, when the inner pipe expands longitudinally during the operation of the engine, respective sections of the outer pipe can move relatively to each other in accordance with the movement of the inner pipe. Since the sliding connection between the outer pipe sections permits relative longitudinal motion between the outer pipe sections, the difference in the amounts of the thermal expansions of the inner and the outer pipes are absorbed by the relative sliding movements of the outer pipe sections, thus stress is not generated in the elements of the exhaust pipe by the difference in the thermal expansions.
In the double walled exhaust pipe disclosed by the related art, the outer pipe is divided into pipe sections which are connected each other by the sliding connections. However, in the double walled exhaust pipe, usually the outer pipe also acts as a structural support for supporting the weight of the inner pipe as well as that of the outer pipe itself. Therefore, the outer pipe is preferably constructed as one piece for strength, and not divided into sections.
To maintain a one piece construction of the outer pipe, the means for compensating for the difference in thermal expansion must be provided on the inner pipe, instead of on the outer pipe. However, it is more difficult to compensate the expansion of the inner pipe properly since the amount of the thermal expansion of the inner pipe is larger than the amount of the expansion of the outer pipe due to higher temperature of the inner pipe during the operation of the engine.
Further, the outer pipe and the inner pipe must be seal welded to each other at the exhaust gas inlet portion to prevent the exhaust gas from penetrating into the radial clearance between the inner pipe and the outer pipe. Namely, the exhaust gas inlet portion becomes a reference point for the expansion of the inner pipe, and the inner pipe expands from that reference point in the direction of the exhaust gas flow. In such a case, the amount of the movement of the inner pipe due to thermal expansion becomes larger as the distance from the exhaust gas inlet portion increases. When the exhaust pipe has a bent portion at downstream of the exhaust inlet portion, this thermal expansion of the inner pipe may cause the deflection of the inner pipe. If the deflection of the inner pipe becomes larger than the clearance between the inner pipe and the outer pipe, the inner pipe and the outer pipe contact at the bent portion. When the contact between the inner pie and the outer pipe occurs, the thermal expansion of the inner pipe is hindered. This may cause the contact noise between the inner pipe and the outer pipe, and in extreme case, cause an excessive thermal stress in the exhaust pipes.